The treadmill will also be more stable if it does not fold in half for storage, as the folding joint on treadmills creates a weak point in the frame. Treadmills with aluminum frames are more stable and durable than those with steel frames because they absorb shock better. This can limit your speed, put excessive strain on your muscles and raise your blood pressure, according the American College of Sports Medicine. If the treadmill shakes while you use it, you are more likely to hold the handrail while exercising. Stability Manual treadmills are much lighter in weight than their motorized counterparts, making it easier for them to wobble during intense walking or light jogging. If the belt is too short or narrow, exercising may be uncomfortable and you will be prone to tripping or stepping off of the belt. While a slightly smaller belt size is acceptable because you will be walking or lightly jogging instead of running full speed on the manual treadmill, you must ensure that there is enough space for your stride length on the belt. Belt Size The walking area on a manual treadmill is typically smaller than that of a motorized treadmill.